The present invention relates to an ion beam generating apparatus.
Along with recent advances in electronics, various new techniques have been developed. A typical example is an ion beam generating apparatus. Since such an ion beam generating apparatus can provide a very small beam spot, it has been receiving a great deal of attention in the field of microelectronics, especially in relation to semiconductor fabrication techniques. In semiconductor fabrication techniques, various types of conventional micropatterning techniques have been proposed. Micropatterning to an accuracy of up to one micron is the current limit in photolithography. The electron beam is no exception. Since the mass of the electron beam is smaller than that of atoms, electron beams are greatly scattered in a semiconductor material in addition to the influence of a proximity effect. On the other hand, an ion beam has a larger mass and so is scattered much less in a material. In other words, an ion beam is free from the proximity effect, thereby allowing a good micropattern to be obtained.
Conventional ion beam generating apparatuses developed so far have a field emission liquid metal ion source. This source is also called an electrohydrodynamic (EHD) ion source. When a needle is wetted with a liquid metal in a high electric field, ions are emitted from the apex of the needle in accordance with a field ionization effect. For example, an ion beam generating apparatus of this type is described by R. Clampitt in "Advances in Molten Metal Field Ion Sources", 1981 Nuclear Instrument and Methods 189 (1981) 111-116, North-Holland Publishing Company. In this case, the metal is heated at a temperature above its melting point since it must be molten. It is very difficult to continuously supply a molten metal to the needle for a long period of time. Various complicated countermeasures must be taken for high temperature treatments.
The present inventor noticed a new technique wherein a gas is supplied to extract, as an ion beam, an element contained in the gas. This technique is reported in the following reference: "H.sub.2 and rare gas field ion source with high angular current", Gary R. Hanson and Benjamin M. Siegel, J. Vac. Sci. Technol., 16(6), Nov./Dec. 1979, .COPYRGT.1980 American Vacuum Society. In this reference, liquid helium is used as a coolant, and hydrogen gas is used as a source gas.
Another reference is "Gas Ion Sources for Micropatterning", Takashi Horiuchi et al, the Japan Society for the Promotion of Science, 132nd Committee, 81st Study Meeting, Sept. 24, 1982, P. 21. In this reference, liquid nitrogen is used as a coolant, and hydrogen gas is used as a source gas.
Since helium and nitrogen, which are liquid in only a narrow temperature range, are used as the coolants, the type of ion able to be selected is limited. In addition, each source gas has an optimal application temperature at which it is preferably set. However, this is not noticed in the conventional apparatuses.